Wavelet depulsing of ultrasound echo sequences

ABSTRACT

Methods and associated apparatuses for imaging a target. An echo sequence image of the target is acquired and a log spectrum of at least a portion of the echo sequence image is computed. A point spread function is estimated by one of two methods. According to the first method, a low-resolution wavelet projection of the echo sequence log spectrum is used as an estimate of the log spectrum of the point spread function. According to the second method, an outlier-resistant wavelet transform of the echo sequence log spectrum is effected, followed by soft-thresholding and an inverse wavelet transform. Under both methods, a frequency domain phase of the point spread function also is estimated, the relevant portion of the echo sequence image is deconvolved using the estimated point spread function.

REFERENCE TO PRIORITY APPLICATION

This application is a national stage application of PCT/US01/13590 filed Apr. 27, 2001. This application also claims priority to U.S. Provisional Application No. 60/203,510 filed May 11, 2000.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to medical imaging and, more particularly, to a method of deconvolving an ultrasonic echo sequence, and to an ultrasound imaging apparatus that employs this method.

Because of the coherence of the back-scattered echo signals, images obtained from echo ultrasound imaging systems have extremely complex patterns that bear no obvious relationship to the macroscopic properties of the insonified object. The vast majority of biological tissues are extremely small on the scale of an acoustic wavelength. Consequently, a signal obtained within a resolution cell consists of contributions of many independent scatterers. Interference of these de-phased echoes gives rise to a pattern that has the appearance of a chaotic jumble of “speckles”, known as speckle noise. The speckle pattern consists of a multitude of bright spots where the interference is highly constructive, dark spots where the interference is destructive, and brightness levels between these extremes. The presence of speckle noise in an ultrasound image reduces the ability of a user to resolve fine details. Speckle noise obscures very small structures, for example, early stage tumors, and decreases the reliability of tissue characterization. Therefore, the suppression of speckle noise is an important component of medical ultrasound imaging.

For the purpose of modeling the interaction of biological tissue with ultrasonic waves, the biological tissue is considered to be an assembly of reflectors and scatterers. A reflector is a plane interface that is large compared to the wavelength and that reflects portions of the transmitted energy back towards the transmitter. A scatterer is an object that is small compared to the wavelength and that scatters the transmitted signal in all directions. Such a system often is modeled as a (most generally 3D) function called the spatial response of insonified material, the reflectivity function, or (in medical applications) the spatial tissue response.

An ultrasound radio frequency (RF) image can be considered to consist of 1D echo sequences, also known as “RF lines”. Assuming the tissue properties to be uniform in the plane perpendicular to the scanning beam, an acquired 2D RF image can be viewed as the result of the convolution of the 2D reflectivity function (which accounts for inhomogeneity in the scanning plane) and the 2D transducer point spread function (PSF). Thus, the RF image can be considered to be a distorted version of the true reflectivity function, where the distorting kernel is the transducer PSF. This distortion includes the speckle noise discussed above.

In principle, it should be possible to measure the PSF in a calibration procedure, and then to deconvolve the PSF from the RF image. In practice, however, this is not possible, for several reasons. Perhaps the most important reason is that the absorption of ultrasound energy in tissues increases with frequency. This frequency-dependent attenuation causes both the PSF amplitude and the PSF shape to depend on depth in the tissue, leading to the observed non-stationarity of RF sequences.

In medical ultrasound, a pulse is transmitted into the tissue to be imaged, and the echoes that are backscattered to the emitting transducer are detected as a voltage trace RF line. The RF line conventionally is modeled as being a convolution of a hypothetical 1D PSF with a hypothetical 1D tissue reflectivity function. Assuming that the scatterers on each image line are located on a uniform grid and that the system impulse response is range shift invariant along each image line, a discretized version of the received signal can be written as: rf[n]=a[n]*s[n]+noise[n]  (1) where n is a time index, rf[n] is the RF line, s[n] is the transmitted ultrasound PSF, a[n] is a reflectivity sequence corresponding to the reflectivity function, noise[n] is measurement noise, and “*” represents convolution. Because the frequency-dependent attenuation process appears as a decrease with distance of the mean frequency and amplitude of the PSF, it is commonly assumed that the received echo signal may be expressed as a depth-dependent PSF convolved with the tissue reflectivity function. To make the PSF “location dependent”, s[n] in equation (1) is replaced by s[n,k], where k is the location index. This leads to the observed non-stationarity of the RF lines received from the tissue. In order to deal with this non-stationarity, the RF-sequence is broken up into a number of possibly overlapping segments, such that within each segment the frequency-dependent attenuation process can be ignored and equation (1) holds. The problem of tissue characterization is thus reduced to a set of blind deconvolution problems: for each segment of a given RF line, the respective ultrasonic PSF should be estimated and removed.

To this end, homomorphic signal processing has been applied to rf[n]. Ignoring the noise term on the right hand side of equation (1) for now, transforming equation (1) to the frequency domain gives: RF(w)=A(w)S(w)  (2) i.e., in the frequency domain, the frequency spectrum A(w) of the reflectivity sequence is multiplied by the frequency spectrum S(w) of the PSF to give the frequency spectrum RF(w) of the echo sequence. These spectra can be written as RF(w)=|RF(w)|e ^(j·arg(RF(w)))  (3) A(w)=|A(w)|e ^(j·arg(A(w)))  (4) S(w)=|S(w)|e ^(j·arg(S(w)))  (5) Taking the complex logarithm of both sides of equation (2) then gives log|RF(w)|=log|A(w)|+log|S(w)|  (6) arg[RF(w)]=arg[A(w)]+arg[S(w)]  (7)

As described in the Annexes, the log spectrum of the echo sequence, log|RF(w)|, thus is a sum of a smooth and regular log spectrum, log|S(w)|, of the PSF and a jagged and irregular log spectrum, log|A(w)|, of the reflectivity sequence. Cepstrum-based techniques have been used to exploit the qualitatively different natures of the PSF and reflectivity log spectra to isolate the PSF log spectrum for the purpose of estimating the PSF and deconvolving the estimated PSF from the echo sequence. See, for example, Torfinn Taxt, “Restoration of medical ultrasound images using two-dimensional homomorphic deconvolution”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 42 no. 4 pp. 543-554 (July 1995); Torfinn Taxt, “Comparison of cepstrum-based methods for radial blind deconvolution of ultrasound images”, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 44 no. 3 pp. 666-674 (May 1997); J. A. Jensen and S. Leeman, “Nonparametric estimation of ultrasound pulses”, IEEE Transactions on Biomedical Engineering, vol. 41 pp. 929-936 (1994); and J. A. Jensen, “Deconvolution of ultrasound images”, Ultrasonic Imaging, vol. 14 pp. 1-15 (1992). Cepstrum-based techniques, however, suffer from certain limitations, as discussed in Annex A. Briefly, the complex cepstrum of a signal of finite duration has been shown to extend to infinity. This invariably leads to aliasing errors when the Discrete Fourier Transform or a similar discrete numerical method is used to compute the cepstrum.

There is thus a widely recognized need for, and it would be highly advantageous to have, a method of estimating an ultrasound PSF that would overcome the disadvantages of presently known methods as described above.

SUMMARY OF THE INVENTION

The present invention includes two innovative methods for estimating the PSF of an echo sequence. The first method is based on a low-resolution wavelet projection of the log spectrum of the echo sequence. The second method is based on the “soft-thresholding de-noising” algorithm of David L. Donaho and his coworkers, modified to account for the fact that the log spectrum of the reflectivity sequence is not normally distributed.

According to the present invention there is provided a method of imaging a target, including the steps of: (a) acquiring an echo sequence image of the target; (b) computing a log spectrum of at least a portion of the echo sequence image; (c) computing a low-resolution wavelet projection of the log spectrum; (d) estimating a point spread function from the low-resolution wavelet projection; and (e) deconvolving the at least portion of the echo sequence image with the point spread function.

According to the present invention there is provided a method of imaging a target, including the steps of: (a) acquiring an echo sequence image of the target; (b) computing a log spectrum of at least a portion of the echo sequence image; (c) effecting an outlier-resistant wavelet transform of the log spectrum, thereby producing a plurality of wavelet coefficients; (d) soft-thresholding the wavelet coefficients; (e) applying an inverse wavelet transform to the soft-thresholded wavelet coefficients to obtain a point spread function log spectrum; (f) estimating a point spread function from the point spread function log spectrum; and (g) deconvolving the at least portion of the echo sequence image with the point spread function.

According to the present invention there is provided an apparatus for imaging a target, including: (a) a transducer for acquiring an echo sequence image of the target; and (b) a processor for: (i) computing a log spectrum of at least a portion of the echo sequence image, (ii) computing a low-resolution wavelet projection of the log spectrum, (iii) estimating a point spread function from the low-resolution wavelet projection, and (iv) deconvolving the at least portion of the echo sequence image with the point spread function.

According to the present invention there is provided an apparatus for imaging a target, including: (a) a transducer for acquiring an echo sequence image of the target; and (b) a processor for: (i) computing a log spectrum of at least a portion of the echo sequence image, (ii) effecting an outlier-resistant wavelet transform of the at least portion of the log spectrum, thereby producing a plurality of wavelet coefficients, (iii) soft-thresholding the wavelet coefficients, (iv) applying an inverse wavelet transform to the soft-thresholded wavelet coefficients to obtain a point spread function log spectrum, (v) estimating a point spread function from the point spread function log spectrum, and (vi) deconvolving the at least portion of the echo sequence image with the point spread function.

According to both methods of the present invention, an echo sequence image of the target is acquired. As understood herein, an echo sequence image is a set of RF lines, acquired in parallel, from which the final video image of the target is to be calculated. Optionally, the echo sequence image is partitioned into a plurality of segments that may be either disjoint or overlapping. Subsequent processing is applied either to the echo sequence image as a whole or to one or more of the segments separately. This processing begins with the computation of the log spectrum of the time series (the whole echo sequence or the segment thereof) being processed.

At this point, the two methods of the present invention diverge temporarily. According to the first method, a low-resolution wavelet projection of the log spectrum is computed. Preferably, this wavelet projection is based on a Coiflet wavelet, on a minimum-phase Daubechies wavelet, on a symmetric Daubechies wavelet or on a biorthogonal wavelet. Optionally, this wavelet projection includes an outlier-resistant wavelet transform of the log spectrum.

According to the second method, an outlier-resistant wavelet transform is essential, not merely optional. Specifically, according to the second method, an outlier-resistant wavelet transform of the log spectrum is effected, and the resulting wavelet coefficients are subjected to the “soft-thresholding” algorithm of David L. Donaho et al., or an equivalent algorithm. The outlier-resistant wavelet transform preferably is based on minimizing an L₁ norm. Most preferably, the outlier-resistant wavelet transform is the Smoother-Cleaner Wavelet Transform of Andrew G. Bruce et al., or an equivalent transform. Robust residuals may be removed at all resolution levels of the outlier-resistant wavelet transform or at only some resolution levels. In particular, robust residuals may be removed at only the first resolution level. An inverse wavelet transform is applied to the soft-thresholded wavelet coefficients, thereby producing a PSF log spectrum.

Either the low-resolution wavelet projection of the first method or the PSF log spectrum of the second method is an estimate of log|S(w)|. The remaining steps of the two methods are identical. An estimate of the frequency domain phase arg[S(w)] of the PSF is obtained, preferably under the assumption that the PSF is a minimum phase sequence. Optionally, a Wiener filter is applied to the estimate of |S(w)|. Equation (5) now gives an estimate of S(w), which is inverse Fourier transformed to give an estimate of the PSF. Finally, the estimated PSF is deconvolved from the time series (the whole echo sequence or the segment thereof) being processed preferably using an approximate inverse.

An apparatus of the present invention includes a transducer for acquiring an echo sequence image of the target and a processor for processing the acquired echo sequence image according to one of the two methods.

Although the present invention is illustrated herein with reference to its primary application to ultrasound imaging, it is to be understood that the scope of the present invention extends to any imaging modality based on receiving echoes of signals transmitted by an impulsive energy source to a target, and to which the principles of the present invention are germane (for example, seismic exploration).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is an overall flowchart of the methods of the present invention;

FIG. 2 is a flowchart of the low-resolution wavelet projection procedure;

FIG. 3 is a flowchart of the second method of estimating log|S(w)|;

FIG. 4 is a high level block diagram of an ultrasound imaging apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of methods, and associated systems, for acquiring an echo sequence image of a target using an impulsive source of energy, and then estimating and deconvolving the point spread function of the impulsive source. Specifically, the present invention can be used to acquire medical ultrasound images with reduced speckle noise.

The principles and operation of echo sequence image acquisition and processing according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, FIG. 1 is an overall flowchart of the methods of the present invention.

In block 10, a 1D ultrasound echo sequence is acquired in the conventional manner.

In block 12, the acquired echo sequence is optionally partitioned into two or more subsequences. As described in Annex A with reference to equation (2.1) (and with a slight change in notation), biological tissue acts as a low pass acoustic filter, so that the frequency content of an ultrasound pulse decreases with time. To compensate for this effect, the echo sequence rf[n] usually is partitioned into a set of subsequences rf[n,k], such that, in each subsequence, the ultrasound PSF s[n,k] can be considered to have a well-defined frequency spectrum. The subsequences rf[n,k] may be either disjoint (i.e., nonoverlapping), or may overlap to a certain extent. In what follows, the subsequence index k will be suppressed for notational clarity.

In block 14, the PSF is estimated. The present invention includes two methods of estimating the PSF. More precisely, the present invention includes two methods of estimating log|S(w)|. According to the first method, which is described in detail in Annex A, a low-resolution wavelet projection of log|RF(w)| is taken to represent log|S(w)|. According to the second method, which is described in detail in Annex B, log|RF(w)| is treated as a noisy representation of log|S(w)|, and the noise is removed using the “soft-thresholding de-noising” algorithm of David L. Donaho and his coworkers, modified to account for the fact that the samples of log|A(w)| are not distributed according to a normal (Gaussian) distribution. Under either method, arg[S(w)] is estimated as described below. Equation (5) then gives an estimate of S(w), whose inverse Fourier transform is the PSF.

In block 16, the estimated PSF is deconvolved from the echo sequence to give an estimate of the reflectivity sequence, as described below.

FIG. 2 is a flowchart of the low-resolution wavelet projection procedure.

In block 20, the wavelet used in the wavelet decomposition of log|RF(w)| is selected. Suitable wavelets include, among others, Coiflet wavelets, minimum-phase Daubechies wavelets, symmetric Daubechies wavelets and biorthogonal wavelets, with Coiflet wavelets being preferred.

In block 22, the resolution level of the wavelet transform that distinguishes log|A(w)| from log|S(w)| is selected. One way of doing this is to calibrate the ultrasound probe by recording the pulse emitted by the probe, either directly or after reflection from a single planar reflector, treating this recorded pulse as a proxy for s[n], computing the corresponding log|S(w)| and determining the number of wavelet coefficients needed to represent this proxy for log|S(w)| with the desired degree of accuracy. As mentioned above, this measured proxy for s[n] can not be used directly for deconvolution of an acquired ultrasound echo sequence, for several reasons. First, the actual s[n] generally is low-pass filtered by the biological tissue. Second, the actual s[n] always differs from the proxy s[n] because of invariable variations in the conditions of acquisition, such as the degree of acoustical impedance mismatch between the ultrasound probe and the target. Nevertheless, the proxy of log|S(w)| can be used to define the resolution level of the wavelet transform because the wavelet resolution levels needed to accurately represent the actual log|S(w)| and the proxy of log|S(w)| generally are identical.

The low-resolution wavelet projection itself is defined in equation (3.10) of Annex A. With a slight change of notation, this equation is: $\begin{matrix} {{{\log{{{RF}(w)}}} = {{A_{2^{J}}\log{{{RF}(w)}}} + {\sum\limits_{j \leq J}{D_{2^{J}}\log{{{RF}(w)}}}}}}\quad} & (8) \end{matrix}$ Note that resolution level decreases with increasing resolution index j. The first term on the right hand side of equation (8), A₂ _(J) log|RF(w)|, is the low-resolution wavelet projection of log|RF(w)| at resolution level J that is computed in block 24. The remainder of the right hand side of equation (8) is the high-resolution portion of log|RF(w)|. A₂ _(J) log|RF(w)| is used in subsequent processing as an estimate of log|S(w)|.

The second method of estimating log|S(w)| is an extension of the “soft thresholding de-noising” algorithm that was developed by David L. Donaho and his coworkers, and that they have described in a series of publications: David L. Donoho, “De-noising by soft-thresholding”, Transactions on Information Theory, vol. 41 no. 3 pp. 613-627 (1995); David L. Donoho and Iain M. Johnstone, “Adapting to unknown smoothness via wavelet shrinkage”, Journal of the American Statistical Association, vol. 90 pp. 1200-1224, (1993); David L. Donoho and Iain M. Johnstone, “Ideal de-noising in an orthonormal basis chosen from a library of bases”, C. R. Acad. Sci. Paris, Series I, vol. 319 pp. 1317-1322 (1994); and David L. Donoho and Ronald R. Coifman, “Translation-invariant de-noising”, Technical Report 475, Department of Statistics, Stanford University, (May 1995). This algorithm, and equivalent algorithms, are referred to herein collectively as “soft-thresholding” algorithms. The purpose of these algorithms is to clean up the noise of a noisy signal in order to extract the signal. In the present context, log|S(w)| is treated as the signal part of log|RF(w)| and log|A(w)| is treated as the noise part of log|RF(w)|: these algorithms are applied to log|RF(w)| to clean up log|A(w)|, thereby isolating and extracting log|S(w)|. These algorithms assume that the noise obeys a Gaussian distribution. It is shown in Annex B that log|A(w)|, treated as random noise, is expected to obey a non-Gaussian distribution, specifically, a Fisher-Tippet distribution. To account for this, the wavelet coefficients of log|RF(w)|, that are input to the soft-thresholding algorithm, are computed using an outlier-resistant wavelet transform. Essentially, wavelet projections are used that minimize the L₁ norm instead-of the L₂ norm at each level of resolution. In practice, for computational efficiency, the “Smoother-Cleaner Wavelet Transform” (Andrew G. Bruce, David L. Donoho, Hong-Ye Gao and R. Douglas Martin, SPIE Vol. 2242 pp. 325-336 (1994)) is used in preference to brute force L₁-norm minimization.

FIG. 3 is a flowchart of the second method of estimating log|S(w)|.

As in the low-resolution wavelet projection of FIG. 2, the first step (block 30) is the selection used in the discrete wavelet transform of log|RF(w)|. The outlier-resistant discrete wavelet transform itself is effected in block 44, as a loop over resolution levels. The loop is initialized in block 32 at the coarsest resolution level. At each resolution level, wavelet coefficients are calculated in block 34, robust residuals are calculated in block 36, and the thus-identified outliers are removed in block 38. In block 40, the loop termination condition is tested: if the finest desired resolution has not been reached, the resolution level is made finer (block 42) and the loop is repeated; otherwise the loop is terminated.

As discussed above and in Annex B, the robust residual calculation of block 36 amounts to computing the wavelet coefficients by minimizing an L₁ instead of an L₂ norm at each level of resolution. This minimization can be done by brute force. It is preferable, however, for numerical efficiency, to use the “Smoother-Cleaner Wavelet Transform” of Bruce et al. for this purpose.

Following the outlier-resistant wavelet transform of block 44, the soft thresholding algorithm of Donaho and his coworkers is applied to the resulting wavelet coefficients in block 46, and the soft-thresholded wavelet coefficients are subjected to an inverse discrete wavelet transform in block 48 to give the final estimate of log|S(w)|.

Although FIG. 3 shows robust residuals being calculated and outliers being removed at all the resolution levels considered, it has been found that it is not strictly necessary to remove outliers at all resolution levels. In fact, it often suffices to remove outliers only at the first (coarsest) resolution level.

It will be appreciated that the outlier resistant wavelet transform of block 44 also may be used in the low-resolution wavelet projection procedure of FIG. 2, in the projection step of block 24.

Optionally, following the exponentiation of the estimate of log|S(w)| obtained by either method, the Wiener filter defined by equation (3.11) of Annex A is used to refine this exponentiated estimate to provide a refined estimate of |S(w)|.

|S(w)| having been estimated, arg[S(w)] now is estimated. As described in Annex A, the preferred estimate of arg[S(w)] is a minimum phase estimate. If it is assumed that the PSF is a minimum phase sequence, then log|S(w)| and arg[S(w)| are a Hilbert transform pair, so that an estimate of arg[S(w)] can be derived from the estimate of log|S(w)|.

With both |S(w)| and arg[S(w)] now estimated, equation (5) gives an estimate of S(w). The inverse Fourier transform of this estimate of S(w) is an estimate of s[n], which is deconvolved from rf[n] by approximate inverse methods, as described, for example, in A. K. Louis, “Approximative inverse for linear and some nonlinear problems”, Inverse Problems, vol. 11 no. 6 pp. 1211-1223 (1995).

FIG. 4 is a high level block diagram of an ultrasound imaging apparatus 50 of the present invention. Apparatus 50 includes a conventional ultrasound transducer 52 coupled to a digital processor 54 via a D/A converter 56 and an A/D converter 58. Processor 54 generates digital ultrasound waveforms that are transformed to analog signals by D/A converter 56 and transmitted into the biological target to be imaged by transducer 52. The consequent ultrasound echoes are received by transducer 52, digitized by A/D converter 58 and processed by processor 54 in accordance with the principles of the present invention to provide images of the target with reduced speckle noise.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. 

1. A method of imaging a target, comprising the steps of: (a) acquiring an echo sequence image of the target; (b) computing a log spectrum of at least a portion of said echo sequence image; (c) computing a low-resolution wavelet projection of said log spectrum; (d) estimating a point spread function from said low-resolution wavelet projection; and (e) deconvolving said at least portion of said echo sequence image with said point spread function.
 2. The method of claim 1, further comprising the step of: (f) partitioning the echo sequence image into a plurality of segments; said computing of said log spectrum, said computing of said wavelet projection, said estimating and said deconvolving then being effected separately for each said segment.
 3. The method of claim 2, wherein said segments are disjoint.
 4. The method of claim 2, wherein at least two of said segments at least partly overlap.
 5. The method of claim 1, wherein said wavelet projection is based on a wavelet selected from the group consisting of Coiflet wavelets, minimum-phase Daubechies wavelets, symmetric Daubechies wavelets and biorthogonal wavelets.
 6. The method of claim 1, wherein said computing of said wavelet projection includes effecting an outlier-resistant wavelet transform of said log spectrum.
 7. The method of claim 1, wherein said estimating of said point spread function includes estimating a frequency domain phase of said point spread function.
 8. The method of claim 7, wherein said phase estimating assumes that said point spread function is a minimum phase sequence.
 9. The method of claim 1, wherein said deconvolving is effected using an approximate inverse.
 10. A method of imaging a target, comprising the steps of: (a) acquiring an echo sequence image of the target; (b) computing a log spectrum of at least a portion of said echo sequence image; (c) effecting an outlier-resistant wavelet transform of said log spectrum, thereby producing a plurality of wavelet coefficients; (d) soft-thresholding said wavelet coefficients; (e) applying an inverse wavelet transform to said soft-thresholded wavelet coefficients to obtain a point spread function log spectrum; (f) estimating a point spread function from said point spread function log spectrum; and (g) deconvolving said at least portion of said echo sequence image with said point spread function.
 11. The method of claim 10, further comprising the step of: (h) partitioning the echo sequence image into a plurality of segments; said computing of said log spectrum, said effecting of said wavelet transform, said soft-thresholding, said applying, said estimating and said deconvolving then being effected separately for each said segment.
 12. The method of claim 11, wherein said segments are disjoint.
 13. The method of claim 11, wherein at least two of said segments at least partly overlap.
 14. The method of claim 10, wherein said outlier-resistant wavelet transform is based on minimizing an L₁ norm.
 15. The method of claim 10, wherein said outlier-resistant wavelet transform is a Smoother-Cleaner Wavelet Transform.
 16. The method of claim 10, wherein said outlier-resistant wavelet transform includes a plurality of resolution levels, with robust residuals being removed in only a portion of said resolution levels.
 17. The method of claim 16, wherein said robust residuals are removed only in a first said resolution level.
 18. The method of claim 10, wherein said estimating of said point spread function includes estimating a frequency domain phase of said point spread function.
 19. The method of claim 18, wherein said phase estimating assumes that said point spread function is a minimum phase sequence.
 20. The method of claim 10, wherein said deconvolving is effected using an approximate inverse.
 21. An apparatus for imaging a target, comprising: (a) a transducer for acquiring an echo sequence image of the target; and (b) a processor for: (i) computing a log spectrum of at least a portion of said echo sequence image, (ii) computing a low-resolution wavelet projection of said log spectrum, (iii) estimating a point spread function from said low-resolution wavelet projection, and (iv) deconvolving said at least portion of said echo sequence image with said point spread function.
 22. An apparatus for imaging a target, comprising: (a) a transducer for acquiring an echo sequence image of the target; and (b) a processor for: (i) computing a log spectrum of at least a portion of said echo sequence image, (ii) effecting an outlier-resistant wavelet transform of said at least portion of said log spectrum, thereby producing a plurality of wavelet coefficients, (iii) soft-thresholding said wavelet coefficients, (iv) applying an inverse wavelet transform to said soft-thresholded wavelet coefficients to obtain a point spread function log spectrum, (v) estimating a point spread function from said point spread function log spectrum, and (vi) deconvolving said at least portion of said echo sequence image with said point spread function. 